A liquid crystal display with a cell (10) is known from EP 0 282 300. The liquid crystal display described therein is formed by two electrode substrate plates and a rim, which connects the substrate plates by their outer edges and at a distance from each other. The inside space of the cell, which is formed by the two substrate plates and the rim, is filled with a nematic liquid crystal material. Each of the two surfaces of the substrate plates facing the liquid crystal material is provided with an orientation layer. According to EP 0 282 300, these layers are formed of a polyimide-silicone derivative. Before the substrate plates are completed for a cell, the surfaces, that later come in contact with the liquid crystal material of the cell, are subjected to a friction step. This has the effect that, when the molecules of the liquid crystal material come into contact with the surfaces of the orientation layer prepared in this way, at least the molecules that are close to the orientation layer align themselves with their longitudinal axes parallel to the friction direction (hereafter called orientation direction). To obtain a steep electro-optical curve, which is further explained below, it is necessary for the liquid crystal molecules, which are arranged between the two liquid boundaries formed by the orientation layers of a finish-installed cell, to have a high torsion angle (also called twist). Torsion angles between 180.degree. and 300.degree. are indicated in EP 0 282 300, while both configuration examples, however, refer to a torsion angle of 270.degree.. To establish this torsion angle, the two substrate plates that form the display are installed with respect to each other so that the orientation direction of the lower substrate plate forms an angle with the orientation direction of the upper substrate plate, which corresponds to the desired torsion angle. In addition, a chiral dopant is added to the liquid crystal material, to establish the torsion angle of the liquid crystal molecules between the liquid boundaries.
As can be derived from the d/p relationship of about 0.65 (d=plate separation; p=pitch), EP 0 282 300 indicates a relationship between the spontaneous twist-pitch Ps and the adjusted twist-pitch Pc, which is between about 0.3 and about -1.0 according to the formula: EQU (Pc-Ps)/Pc.
EP 0 259 822 explains in greater detail what is meant by spontaneous twist-pitch and adjusted twist-pitch.
However, this document refers to displays whose relationship between spontaneous and adjusted twist-pitch according to the above formula is between 0 and 0.3, thus a display in which the addition of the doping medium produces an overdose with reference to the torsion angle of the molecules between the substrate plates (equal to the values of the above formula, which are positive).
In EP 0 282 300, a polarizer is connected to the respective substrate plate on each side of a cell facing away from the liquid crystal material. The polarization direction of each of the two polarizers is attuned to the orientation direction of the substrate plate, to which it is connected. The angle relationships between the polarization direction and the orientation direction are between 30.degree. and 70.degree. while the angle relationship of the configuration examples is close to 45.degree..
The effect of a cell constructed in this manner, in relation to the independence of the angle of view of contrast and white impression, is achieved in that the product of plate separation and anisotropy of the angle of refraction .DELTA.n of the liquid crystal material is in the range between 0.3 and 0.7 .mu.m, while both configuration examples in EP 0 282 300 refer to values of 0.55 or 0.59 .mu.m.
Whether, or which approach angle (also called pretilt) is assumed by the liquid crystal molecules with respect to the surface of the orientation layers, cannot be found in EP 0 282 300. EP 0 376 029, a document that is also concerned with highly twisted liquid crystal displays of the above described type, indicates valid approach angles that are greater than 7.degree.. Deviating from EP 0 282 300, EP 0 376 029 indicates the product of plate separation and anisotropy of the angle of refraction .DELTA.n as smaller than 0.6 .mu.m, where the preferred value range is between 0.35 and 0.45 .mu.m. EP 0 376 029 indicates torsion angles greater than 240.degree., although it points out that for reasons of contrast, multiples of 90.degree., therefore 270.degree. and 360.degree. are preferred torsion angles Accordingly, the configuration examples in EP 0 376 029 refer to torsion angles of 270.degree. and 360.degree.. Most of the thus constructed displays, called EVA cells, are neutral in color, i.e. in the transmissive condition of the display, white light emanates from the cell across a wide angle of view, while the cell is black in the non-transmissive condition.
In view of their multiplex rate, the quality of passively controlled liquid crystal displays is generally expressed by the electro-optic curve, where significance is given to the difference in the control voltage, to switch a display between 10 and 90% transmission. In this connection, FIG. 3 of EP 0 376 029 indicates a curve which shows, after correction of the drawing (the percentages of the ordinates do not refer to absorption, but erroneously to transmission), that 10% transmission is achieved with a control voltage of 2.95 volts, and 90% transmission with 2.80 volts. However, these transmission values are only valid when an increasing voltage is used, for example. If one starts with a predetermined control voltage value, and the control voltage decreases, 90 or 10% transmission is achieved with other voltages than if the voltage increases. This can be attributed to the fact that liquid crystal displays with torsion angles somewhat greater than 240.degree. have no equal relationship of the determined transmission values with reference to increasing and decreasing control voltage values. In other words, liquid crystal displays of this type, and with torsion angles somewhat greater than 240.degree., are no longer free of hysteresis
To eliminate the hysteresis problem, it is known from EP 0 376 029 to adjust the relationship between the elastic constant K.sub.3 and the elastic constant K.sub.1 to a value between 0.9 and 1.5. These indications are only valid for displays whose Pc to Ps relationship is between 0 and 0.3, determined by the above formula, thus exhibiting excess doping with reference to the torsion angle of the molecules in the plate condition. It should further be pointed out that with a display according to EP 0 259 822, no black-white display is possible while maintaining the approach angle of about 30.degree. mentioned in the configuration examples, only switching between color contrasts.
Furthermore, with highly twisted cells (torsion angle greater than about 240.degree.) it is not possible to produce gray scales in a simple manner, that is, for example to obtain pixel control that permits assigning a determined transmission value to each control voltage value with increasing control voltage.
See FIG. 4 for an explanation in greater detail of the last two aspects (hysteresis and gray scale problems), which schematically illustrates the relationship between the control voltage and the determined transmission of a liquid crystal display with a torsion angle of 270.degree. according to the state of the art (EP 0 376 029). If a control voltage is applied to the electrodes of this liquid crystal display, a transmission value is assigned to each voltage value up to about 2.4 volts, as clearly shown by line 40a. If the voltage is further increased to about 2.5 volts, the transmission drops vertically from about 75% to about 5%. This is illustrated by the dash-dotted line 41. The arrow pointing downward next to line 41 makes it clear that line 41 indicates the relationship between increasing control voltage and transmission. If the control voltage is increased above 2.5 volts, the relationship between increasing control voltage and transmission turns to the right, along line 40b. In the inverse case, namely when a control voltage of about 3 volts is applied to the electrodes of the liquid crystal display and the control voltage is reduced, a transmission value is assigned to each voltage value up to about 1.6 volts, as illustrated by line 40b. If the control voltage is further reduced to about 1.5 volts, a sudden increase from about 25% to about 95% takes place in the transmission. This is shown by the vertical broken line 42 which runs upward. The arrow left of line 42 pointing upward makes it clear that line 42 applies to decreasing control voltages. If the control voltage is reduced to below 1.5 volts, the relationship between transmission and control voltage turns to the left, along line 40a.
This phenomenon can be attributed to the fact that, with an increase in the torsion angle, the calculated curve takes a more S-shaped form. Such an S-shaped curve is shown in FIG. 4, and is composed of lines 40a, 40b and 40c. Complicated measures, or measures that deteriorate the display, are known to still provide gray scale capability to such liquid crystal displays, whose curves correspond to the type shown in FIG. 4.
Thus, for example, a pixel intended for display can be subdivided into 4.times.4 subpixels, for example. Sixteen gray scales are possible with the corresponding switching of these subpixels. Aside from the poor resolution of such displays, these measures are subject to limits created by the producibility of such displays.
Another possibility of creating gray scales with highly twisted liquid crystal displays consists in drastically reducing the switching time between light passage and blocked light passage per unit of time, because the more frequent switching that takes place between a light and dark condition during a unit of time, the more gray scales are possible. In view of the relationships known today, it seems that if the picture repeatability rate is at 50 Hz for example, shortening the switching times seems to be very costly at least, if not altogether impossible.
For that reason, the invention has the task of indicating a liquid crystal display, which permits the imaging of gray scales in a simple manner, with extraordinarily low dependence of the contrast and the white impression on the angle of view, or in which a clear relationship exists between control voltage and transmission.